Genesis 1:21
“וַיִּבְרָא אֱלֹהִים, אֶת-הַתַּנִּינִם הַגְּדֹלִים; וְאֵת כָּל-נֶפֶשׁ הַחַיָּה הָרֹמֶשֶׂת אֲשֶׁר שָׁרְצוּ הַמַּיִם לְמִינֵהֶם, וְאֵת כָּל-עוֹף כָּנָף לְמִינֵהוּ, וַיַּרְא אֱלֹהִים, כִּי-טוֹב”
“God created the great sea monsters and every living creature that moves, with which the waters swarmed after their kind, and every winged bird after its kind; and God saw that it was good.”

On the fifth day, God created the creatures of the sea, including the sea monsters.
The word for ‘mosters’ ‘תַּנִּינִם’ is missing the letter י Yod that makes it proper plural form. The Zohar explains that this is the secret of the Leviathan and his female but because of their great powers, God killed the woman and castrated the male so they won’t corrupt the spiritual balance in the world. God preserved that energy for the Tzadikim in the world to come.